1. Field of the Invention
The present invention concerns a pruning system for rows of fruit trees.
As is known, traditional mechanical pruning (using single-layer pruning machines) was introduced in Italy in the Seventies to facilitate manual pruning operations, which at that time required more than 200 hours per hectare.
2. Present State of the Art
Mechanical pruning was carried out with different types of machinery (with cutter bar, with double or single blade, with knives, disks, etc.) mounted in front of a tractor and required approximately 10-20 hours per hectare.
The single-layer pruning machines used at present make it possible to vary the inclination of the vertical cutting plane from 0 to approximately 15 degrees. In this way, the upper portion of the trees is narrower (approximately 80 cm) and this allows the lower portion of the planting row, which is wider at the base (approximately 110 cm), to receive more light.
In modern fruit plantations the vertical cutting operation and the cutting of the row tops (horizontal cutting) requires only 3 hours per hectare at a speed of 2 km/h. In this way a very narrow row wall is obtained, which is called “fruit-bearing wall” and offers the following advantages:                better exposure of fruits to light;        30-40% reduction in manual pruning times;        elimination of plant growth regulators (PGRs), the plant hormones used to keep the plant branches shorter;        improved access to the fruit trees with any type of machinery and simplification of cultivation operations (branch bending, manual thinning of branches, fruit picking);        reduction in the volumes of water necessary for phytosanitary treatments.        
With the single-layer pruning machines used at present, the cutting operation is carried out along the planting row, substantially on two dimensions, height and length.
Consequently, said pruning machines do not get into the vegetation of the planting row but remain on the surface of the same, at a distance of approximately 50-70 cm from the trunk, forming a single cutting plane near the vertical axis, if necessary associated with a horizontal cutting plane at the level of the tree tops and with a further cutting plane at approximately 50 cm from the ground.
The only degrees of freedom of said pruning machines are represented by the distance from the trunk and the inclination of the vertical cutting plane.
Consequently, the cutting operation performed by said pruning machines creates a sort of “hedge” in the shape of a frustum of pyramid, which requires a manual finishing operation consisting in the elimination of the excess wood present inside the plant.
Consequently, a planting row with very limited thickness, preferably less than 1 metre, is obtained, which forms a sort of “fruit-bearing wall”.
“Multilevel cutting” (Van de Vrie, 1973), instead, produces a cut close to the horizontal plane, starting from the outside and getting into the plant, but it is suitable for vigorous trees with rising branches, typical of forms of cultivation that are not used any longer in modern systems.
This type of cutting operation, however, is carried out with several passes and therefore becomes a complex operation that requires a certain amount of time.
Another prior are document is the patent application DE-A-3402801,which disclose a movable hedge-cutting device supported by a rod suited to be moved in orthogonal direction with respect to the direction of the plant row.
A further prior art document is the patent application US-A-4355497, which discloses an apparatus for the trimming trees including a self-levelling cutter supporting frame and a number of knive-like cutting blades mounted on the frame for rotation at high speed.